Memory CD8+ T cells provide essential specific immunity against previously encountered antigens, and formation of such cells is a fundamental goal of vaccination. Our data show that the pre-immune population of CD8 T cells contains cells with memory-like properties, which we call Homeostatic memory (HM) cells. In this proposal we will test how this population of T cells contributes to primary immune responses. Building on our previous data, we will conduct two aims. 1) how the HM pool responds to pathogens early during an immune response, and how these cells may affect pathogen growth or immunopathology that can arise during immune responses. We will also extend data suggesting HM cells differ qualitatively from nave cells in their response to antigen. 2) This aim extends preliminary data suggesting the cytokine IL-4 plays an unexpected role in controlling CD8 T cell responses. We will assess the function of IL- 4 in controlling memory CD8 T cell maintenance, and build on our preliminary data showing that IL-4 supports the CD8 response to pathogens, by determining the stage at which IL-4 mediates its effects.